Wood is a biodegradable material which, at moisture contents ranging from 20-40% (Colley & Rumbold 1930, Cartwright and Findlay 1958, Soderstrom 1986), is susceptible to fungal attack. Sapstain is the name given to the greyish, blackish or blueish discolouration of the sapwood resulting from the presence of pigmented fungal hyphae that penetrate along the medullary ray cells. Sapstain fungi utilise the easily assimilable nutrients in the wood leaving the structural carbohydrates. Sapstain is thus considered to be an aesthetic problem causing negligible loss of biomass or strength properties of the wood. The presence of these fungi in the wood, however, creates favourable conditions for infection of decay fungi and some sapstain fungi have been known to cause soft rot (Wang & Zabel 1990).
Three broad groups of fungi are associated with sapstain; 1) Ophiostomatalean fungi including species of Ceratocystis, Ceratocystiopsis and Ophiostoma, 2) Black yeasts such as Hormonema dematioides, Rhinocladiella atrovirens, Aureobasidium pullulans, Leptodontidium elatius, and Phialophora spp. 3) Dematiaceous moulds such as Alternaria alternata and Cladosporium cladosporoides (Seifert 1992). Growth of other surface moulds such as Penicillium and Trichoderma spp, while frequently producing abundant green conidia on wood, discolours only the surface of the wood and can be easily removed by planing.
Sapstain fungi can be controlled by protective chemicals. In British Columbia, Canada alone, mill practices involve the annual treatment of 3.6 million board feet of lumber with a value in excess of Can. $2 billion (Gilbert 1988). Without chemical treatment, a significant portion of high value lumber must be sold in lower value markets at an estimated potential cost to the British Columbia lumber industry of Can. $388 million per annum (Deloitte et al, 1989). For over 50 years the industry relied on chlorinated phenols to control fungal growth but concerns over carcinogenicity, fish toxicity and the presence of potential dioxin contaminants resulted in discontinuance the use of these chemicals (Bray 1981, Jones 1981). Most of the alternative chemicals are not as universally effective as the chlorinated phenols (Miller & Morrell 1989, Miller et al, 1990) and also result in high fish toxicity, eye and skin sensitivity (Henderson 1992, Hanssen et al, 1991) corrosion of equipment, unwanted discoloration of wood and increased costs (Gilbert 1988).
One of the long term objectives of the sawmill industry is to eliminate the use of toxic substances for the protection of lumber against stain, mould and decay. There has been much interest in biological control for agriculture applications, and some products have been registered (Lewis et al, 1991). However, the majority of efforts have shown that biological control is both less efficacious and more variable than control obtained with chemical agents (Harman and Lumsden 1990). A principal reason for these suboptimal results is the poor germination and growth of the biocontrol agent.
A number of biological control organisms (bioprotectants), including bacteria, mycorrhizal fungi, decay fungi and other sapwood inhabiting fungi have been investigated Benko 1988, 1987, Selfeft et al, 1988, Croan & Highley 1990, Morrell & Sexton 1992) but no successful applications in wood have been developed. Several strains of the genus Gliocladium have been reported to have utility in the protection of lumber against sapstain on small blocks of wood, for example in Applicant's Patent Cooperation Treaty application number PCT/CA92/00299 that was published under number WO 93/01923 on Feb. 4, 1993.
The mechanisms by which biological control organisms (bioprotectants) achieve control are thought to be through interference competition (mycoparasltism, inhibitory or toxic metabolite production) or by exploitation competition (competition for nutrients). The microbial community (or microflora) of lumber is composed of populat ions of fungi, yeasts, bacteria and actinomycetes. The species composition of the microflora is a result of the nutrient availability in the substrate as well as climate and seasonal factors, substrate species and the spores circulating in the air surrounding the lumber. The interact ions between an introduced bioprotectant isolate and the existing community will be strongly influenced by all of the above factors. Any method of controlling sapstain in lumber will have to take into account the presence of the microbial flora in the lumber in order to be effective.
Another recognized problem with wood products in several countries is the presence of the pinewood nematode (PWN). In Japan the PWN has been shown to be responsible for pine wilt disease (Mamiya and Kiyohara. 1972), resulting in destruction of some plantation grown pine species. Pine wilt disease has never been recognized as occurring in Canada's forest, where the necessary conditions for the development of this disease do not seem to occur (Rutherford et al 1990). The PWN also occurs in China (BaoJun and Quoli 1989), Taiwan and Korea. The European Community (EC) believes that the PWN does not occur in Europe. Since the EC is very dependent on the importation of lumber and timbers, particularly from North America, they have become very concerned about the potential introduction of PWN to Europe. Canada has been very responsive to the EC concerns and has, over the years, implemented measures to dramatically reduce the risk of exporting PWN. A very strict lumber grading inspection program known as the Mill Certification Program for Bark and Grubhole Control (MCP), has been implemented. Through this program, sawmills and shippers guarantee that all bark and grubholes have been eliminated from the lumber. It is worth noting that following almost 100 years of lumber shipments and 200 years of round wood shipments such as masts as spars, from North America to the EC, even without the MCP in place, no evidence of pine wilt disease has been found in Europe.
In view of the above problem with pinewood nematodes, the Applicant was commissioned by Forestry Canada to lead a research initiative to examine pasteurization as an alternative to kiln drying for the eradication of the PWN from lumber.
Pasteurization uses temperatures lower than those required for sterilization, resulting in the killing of select-organisms. It can be used where higher temperatures may be detrimental to the materials being heated, as for example in the pasteurization of milk or in the brewing industry. It has been used to control a range of microorganisms in various materials, including nematodes in soil (Todd and Pearson, 1988). The use of simple and clean, wet heat, is very attractive and presents no direct environmental problems. It is a well established fact that PWN (Dwinell 1990) and beetles (Ostaff and Cech. 1978), like most organisms, can be killed by moderate heat, but for Canadian lumber species little data exists on what temperature would be required, or for how long.
Applicant determined that the pasteurization of unseasoned coniferous wood under laboratory conditions using wet heat at 56.1.degree. C. for 30 minutes, resulted in total mortality for PWN with a reliability of 99.994% with 95% confidence. This conclusion was derived for the worst conditions of PWN isolate, wood species and moisture content of those tested.
Pasteurization of unseasoned lumber, using an operational temperature of 56.degree. C. for 30 minutes, was demonstrated at three different locations across Canada using a conventional, high temperature, and dehumidification kiln. No surviving PWN were found in the lumber treated at any of the three locations. This clearly demonstrated the applicability of pasteurization under mill conditions using a temperature slightly above that found to be necessary under laboratory testing.
However it was recognized that before pasteurization could be applied to industrial lumber production its deleterious effect on the antisapstain formulations used to protect lumber in transit and storage would have to be evaluated. In a study by Clark and Smith (1990) it was determined that lumber that has been pasteurized is more susceptible to re-infection and discoloration by staining and mold fungi than wood that has not been pasteurized. The authors therefore concluded that pasteurized wood will require antisapstain treatment to prevent degradation by stain, mold and decay fungi.
A recent study by Byrne and Minchin (1992) clearly shows the magnitude of this problem and its specificity to individual formulations. In particular they noted that pasteurization reduced the efficacy of chemical ant i-sapstain agents and concluded that pasteurized wood was a greater challenge to sapstain control.